Vibration spectra of molecules adsorbed on deformed metals

Abstract

A quantum-chemical cluster model for calculating the vibration spectra of water and hydrogen sulfide molecules adsorbed on nickel and copper is developed and substantiated. The effect of noticeable shifts of the major vibration peaks during deformation of metals is demonstrated.The universality of the phenomenon of the adsorptive effect of a medium on the mechanical properties of solids (the Rebinder effect) and its fundamental importance in solving practical problems made it necessary to thoroughly study its nature.From the standpoint of thermodynamics, this effect is caused by a decrease in the interfacial surface energy at a solid-medium interface [ 1 ]. On the other hand, real patterns of deformation and breakdown of the lattice of a solid, nucleation and propagation of cracks with the participation of surface-active particles of the medium were reproduced by the method of molecular dynamics [3] with the use of a detailed numerical analysis at the atomic-molecular level. However, the application of the concepts of molecular mechanics with complete replacement of the complex energy interaction by simple interatomic potentials does not allow one to consider, in this method and in similar ones, the chemical character of the influence of adsorbed particles and the surface of the solid on the structure and strength of bonds between them.By employing the quantum-chemical semiempirical cluster methods, we calculated the effect of elastic deformation on the adsorption energy and structure of surface complexes. This enabled us to establish significant activation of the chemisorption process [3,4]. However, the obtained results, despite a satisfactory qualitative agreement with experimental data, do not offer a detailed quantitative explanation of the real dynamics of surface phenomena occurring in mechanical force fields. Therefore, taking quantum-chemical cluster calculations as a base, we must study the dynamic characteristics of particles chemisorbed on the surface of a solid under deformation of the crystalline lattice. Among these characteristics, vibration spectra of simple molecules are the most available.To simulate vibrations, we chose the following simple chemisorption systems: water and hydrogen sulfide molecules on nickel and copper clusters. Calculations were performed with an IBM 486 DX4-100 personal computer by a specially parametrized (for obtaining spectra) semiempirical method ZINDO/S [5]. Clusters were composed of 2, 4, and 11 atoms of the metal, which simulate the surface (111) (Fig. 1). Since a change in the electronic structure is rather local during chemisorption, the calculation of small clusters reproduces it rather adequately [6]. A quantum-chemical approach is expedient when the major characteristics of the vibration spectrum can be obtained for small clusters. Then details can be refined in calculations with large clusters. Here, we deal with actually short-range chemical forces. Preliminary calculations of an equilibrium geometry were performed for simple systems (Table 1). Both molecules ...